1. Field of the Invention
The present invention relates to thiol terminated compounds and, more specifically, to gold nanoclusters capped with a thiol terminated compound.
2. Description of the Prior Art
The use of thiol terminated molecules for the formation of self-assembled monolayers on gold surfaces is finding increasing use in nanoscale device fabrication. The majority of these uses have focused on long (twelve to eighteen carbon) alkanethiols, which are commercially available and can be used to passivate the gold surface or define lines and patterns on the micro- to nanoscale. While suitable for these basic applications, the lack of reactivity of the alkane chain limits the use of such structures for further assembly or structure fabrication. This has led to the synthesis of new thiols containing functional groups that are at opposite ends of the alkane chain, e.g., α, ω-thiol carboxylic acids. These molecules use the thiol for attachment to the gold surface, exposing the second reactive group for further assembly. Complementary to this approach of chemically altering the ends of the molecule is the possibility of altering the structure of the carbon chain itself from one that is hydrophobic to one that is hydrophilic. The syntheses of such nonionic hydrophilic alkylthiol surfactants could provide very important source materials to use in these self-assembled films and also find application in biotechnology and chemical sensing.
Another useful material configuration of the alkanethiol monolayer-gold combination is the monolayer deposited on a gold nanoparticle. This is referred to as a stabilized nanocluster, and metal nanoclusters of this nature are currently of interest as building blocks for applications in nanoelectronics. The monolayer encapsulated metal nanocluster is a unique material composed of a metal core and an insulating organic shell. As such, it possesses properties of both metals and insulators. These properties are strongly dependent on the dimensions of the core diameter and the shell thickness. Electrical and optical properties are derived from the metal core of the cluster. The larger the core, the higher the electrical conductivity and optical absorbance. The organic shell is composed of a monolayer of ligand molecules bonded to the surface of the metal core. The shell stabilizes the cluster against irreversible aggregation, promotes solubility/processability in a wide variety of solvents, modulates the electrical conductivity by its relative thickness and dielectric constant, and is the region where chemistries of ligand exchange and self-assembly occur. Procedures for the preparation of these clusters from gold salts and both charged and neutral ligand molecules have been described in the following references: U.S. Pat. No. 6,221,673; U.S. Pat. No. 6,025,202; COLLOIDAL GOLD: PRINCIPLES, METHODS, AND APPLICATIONS 1, 13-32 (M. A. Hayat ed., 1989); and Andrew N. Shipway, Eugenii Katz & Itamar Willner, Nanoparticle Arrays on Surfaces for Electronic, Optical, and Sensor Applications, CHEMPHYSCHEM 1, 18-52 (2000). In addition to modulation of properties, the design of the ligand molecule determines the processability, hence the utility of the cluster material. Currently, well-known ligand molecules used to stabilize gold clusters include alkanethiols of various chain lengths (Mathias Brust, Merryl Walker, Donald Bethell, David J. Schiffrin & Robin Whyman, Synthesis of Thiol-derivatised Gold Nanoparticles in a Two-phase Liquid-Liquid System, J. CHEM. SOC., CHEM. COMMUN. 1994, 801-02), alkylamines of various chain lengths (Daniel V. Leff, Lutz Brandt & James R. Heath, Synthesis and Characterization of Hydrophobic, Organically-Soluble Gold Nanocrystals Functionalized with Primary Amines, LANGMUIR 12, 4723-30 (1996)), phosphines (Gunter Schmid, Metal Clusters and Cluster Metals, POLYHEDRON 7, 2321-29 (1988); Walter W. Weare, Scott M. Reed, Marvin G. Warner & James E. Hutchison, Improved Synthesis of Small (dCORE≈1.5 nm) Phosphine-Stabilized Gold Nanoparticles, J. AM. CHEM. Soc. 122, 12890-91 (2000)), citrates (Katherine C. Grabar, R. Griffith Freeman, Michael B. Hommer & Michael J. Natan, Preparation and Characterization of Au Colloid Monolayers, ANAL. CHEM. 67, 735-43 (1995)), and alkanethiols terminated by ionic or ionizable groups (e.g. sodium sulfonate, carboxylate, etc.) (Marvin G. Warner, Scott M. Reed & James E. Hutchison, Small, Water-Soluble, Ligand-Stabilized Gold Nanoparticles Synthesized by Interfacial Ligand Exchange Reactions, CHEM. MATER. 12, 3316-20 (2000); Sihai Chen & Keisaku Kimura, Synthesis and Characterization of Carboxylate-Modified Gold Nanoparticle Powders Dispersible in Water, LANGMUIR 15, 1075-82 (1999)).
One area where an opportunity is seen for these nanoclusters to make an impact is chemical sensors owing to the ease of fabrication of such devices, and several examples of nanocluster-based sensors have already been reported. Hank Wohltjen & Arthur W. Snow, Colloidal Metal-Insulator-Metal Ensemble Chemiresistor Sensor, ANAL. CHEM. 70, 2856-59 (1998); N. Cioffi, I. Losito, L. Torsi, I. Farella, A. Valentini, L. Sabbatini, P. G. Zambonin & T. Bleve-Zacheo, Analysis of the Surface Chemical Composition and Morphological Structure of Vapor-Sensing Gold-Fluoropolymer Nanocomposites, CHEM. MATER. 14, 804-11 (2002); Li Han, David R. Daniel, Mathew M. Maye & Chuan-Jian Zhong, Core-Shell Nanostructured Nanoparticle Films as Chemically Sensitive Interfaces, ANAL. CHEM. 73, 4441-49 (2001); Andrew N. Shipway, Michal Lahav, Ron Blonder & Itamar Willner, Bis-Bipyridinium Cyclophane Receptor—Au Nanoparticle Superstructures for Electrochemical Sensing Applications, CHEM. MATER. 11, 13-15 (1999); Andrei B. Kharitonov, Andrew N. Shipway & Itamar Willner, An Au Nanoparticle/Bisbipyridinium Cyclophane-Functionalized Ion-Sensitive Field-Effect Transistorfor the Sensing of Adrenaline, ANAL. CHEM. 71, 5441-43 (1999); Michal Lahav, Rachel Gabai, Andrew N. Shipway & Itamar Willner, Au-colloid-‘molecular square’ superstructures: novel electrochemical sensing interfaces, CHEM. COMMUN. 1999, 1937-38; Michal Lahav, Andrew N. Shipway & Itamar Willner, Au-nanoparticle-bis-bipyridinium cyclophane superstructures: assembly, characterization and sensoric applications, J. CHEM. Soc., PERKIN TRANS. 2, 1925-31 (1999); Michal Lahav, Andrew N. Shipway, Itamar Willner, Mogens B. Nielsen & J. Fraser Stoddart, An enlarged bis-bipyridinium cyclophane-Au nanoparticle superstructure for selective electrochemical sensing applications, J. ELECTROANAL. CHEM. 482, 217-21 (2000); Agnes Labande & Didier Astruc, Colloids as redox sensors: recognition of H2PO4− and HSO4− by amidoferrocenylalkylthiol-gold nanoparticles, CHEM. COMMUN. 2000, 1007-08; So-Jung Park, T. Andrew Taton & Chad A. Mirkin, Array-Based Electrical Detection of DNA with Nanoparticle Probes, SCIENCE 295, 1503-06 (2002); Stephen D. Evans, Simon R. Johnson, Yaling L. Cheng & Tiehan Shen, Vapour sensing using hybrid organic-inorganic nanostructured materials, J. MATER. CHEM. 10, 183-88 (2000); Qing-Yun Cai & Edward T. Zellers, Dual-Chemiresistor GC Detector Employing Monolayer-Protected Metal Nanocluster Interfaces, ANAL. CHEM. 74, 3533-39 (2002). Many of these studies utilize alkanethiol-stabilized clusters, which are readily synthesized in a variety of core and shell sizes, are stable, charge neutral, and able to undergo thiol substitution reactions which facilitate their self-assembly into nanostructures. Despite these advantages, the solubility of such clusters is limited to organic solvents, and it is not difficult to envision situations where aqueous solubility is required, e.g., in DNA based assembly of nanostructures.
Most work involving the use of aqueous gold clusters uses the well-known citrate stabilized gold colloids. These clusters utilize ionic interactions on the cluster surface to obtain solubility, and as a result agglomerate irreversibly on removal of the solvent. They are also larger in size (˜12 nm) than the typical alkanethiol stabilized cluster (<2 nm). Reports of water-soluble thiol-coated gold nanoparticles are limited, and most of these clusters are stabilized by an alkanethiol terminating in either an ionic species or a carboxylic acid group. As a result, the aqueous solubility of these clusters is often dependent on the pH of the solution, and the presence of functional groups which can participate in hydrogen bonding between clusters can lead to particle agglomeration. Additionally, the presence of ions in the system leads to interference during conductivity measurements and electron transport studies. Ionic effects are of particular concern in nanoelectronics applications where they can accentuate threshold nonuniformities associated with background charges.
One interesting example of a charge-neutral, non-ionizable water-soluble cluster has been reported where a poly(ethylene glycol) chain was attached to the surface of a gold nanocluster. W. Peter Wuelfing, Stephen M. Gross, Deon T. Miles & Royce W. Murray, Nanometer Gold Clusters Protected by Surface-Bound Monolayers of Thiolated Poly(ethylene glycol) Polymer Electrolyte, J. AM. CHEM. Soc. 120, 12696-97 (1998). While this cluster possesses aqueous solubility, the extreme size of the ligand (a polymer of MW=5000) results in an inability for the cluster to undergo thiol-exchange reactions. This is a serious drawback that limits the utility of this material since such reactions are required to perform the self-assembly of clusters onto devices, a necessary step in the further study of these materials as nanoscale building blocks. A second drawback to this material is that it lacks appreciable conductivity without ion doping, limiting its role in a sensor or nanoscale electronic device.